SiliconâOxygen Network

J. Phys. Chem. B 2009, 113, 11865–11875

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Covalently Bonded Assembly of Lanthanide/Silicon-Oxygen Network/Polyethylene Glycol Hybrid Materials through Functionalized 2-Thenoyltrifluoroacetone Linkage Xiao Fei Qiao and Bing Yan* Department of Chemistry, Tongji UniVersity, Siping Road 1239, Shanghai 200092, China ReceiVed: February 24, 2009; ReVised Manuscript ReceiVed: July 21, 2009

2-Thenoyltrifluoroacetone (TTA) used as the organic ligand and the poly(ethylene glycol) (PEG400 with the molecular weight of 380-430) used as the network precursor were grafted onto the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC), respectively, to construct two precursors TTA-Si and PEG-Si. Then the precursor TTA-Si and the terminal ligand 1,10-phenanthroline (Phen) have coordinated to the rare earth ions by the carbonyl group or nitrogen atom to obtain binary or trinary hybrid polymeric materials after hydrolysis and copolycondensation between the tetraethoxysilane (TEOS), water molecules, and the network precursor PEG-Si via the sol-gel process. The terminal ligand 1,10-phenanthroline (Phen) was used to investigate the difference of photophysical and luminescent properties between binary and trinary hybrid materials, and the network precursor PEG-Si was induced to show its influence on microstructure and thermal properties. The results have revealed that the hybrid materials containing organic ligands bonded with PEG400 showed more efficient intramolecular energy transfer between the europium ion and the ligands (TTA-Si and Phen) and more excellent characteristic emission of the europium ion under UV irradiation with higher 5 D0 luminescence quantum efficiency than the hybrid materials without PEG400, while less uniformity in the microstructure. 1. Introduction The trivalent lanthanide ions have been known for their unique optical properties such as sharp linelike emission spectra and a wide range of luminescence quantum efficiency based on f-f transition.1-3 Since the f-f electronic transitions are forbidden, the complexation of these ions with organic ligands, such as β-diketones, aromatic carboxylic acids, and heterocyclic derivatives, offers several advantages to design efficient lightconversion molecular devices (LCMD).4,5 In particular, some lanthanide complexes with β-diketones have been found to show laser action in solutions.6,7 In these organic complexes, ligands could absorb light energy, transfer it to the emitting metal ions (the so-called antenna effect) due to the large absorption coefficient of the ligand,8,9 and also replace the water molecules around emitting ions to avoid the luminescence quenching effect. However, there exist some restrictions for the practical application of these complexes, so they have to be induced into some matrix, which could be sol-gel glasses,10-12 inorganic-organic hybrid materials,13,14 polymers,15-17 or liquid crystals.18-20 According to a number of investigations in the past few years, the conventional doping method by a sol-gel procedure seems unable to solve the problems of clustering of the emitting center, inhomogeneous dispersion of two phases, or leaching of the photoactive molecules due to the weak interactions (such as hydrogen bonding, van der Waals force, or weak static effect) between organic and inorganic components.21 Since these problems lead to the lower concentration of the lanthanide complexes, many scientists have focused their attention on another kind of hybrid material, where the lanthanide complexes and matrices are connected by covalent bonds through the sol-gel process, and the molecular-based hybrid materials derived from this connection method show improved chemical * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +86-21-65982287. Phone: +86-21-65984663.

stability and exhibit monophasic appearance even with the high concentration of lanthanide complexes.22-31 The sol-gel process has been identified as an effective method based on cohydrolysis and copolycondensation reactions, embedding the lanthanide compounds into the inorganic matrices to achieve the unification between the photophysical properties of the lanthanide complex and the stable characteristics of the inorganic Si-O network. Due to the low processing temperature in the sol-gel procedure, the active groups (lanthanide complex component) could be incorporated with the inorganic matrices at room temperature so the microstructure, the external shape, and the degree of combination between the two components could be controlled by altering the sol-gel processing conditions.32,33 Furthermore, this kind of hybrid material connected by covalent bonds mentioned above merely contains the inorganic Si-O network and the small organic ligand, not the real polymer within the organic chemistry region. So there exist some disadvantages of their application for the optical devices owing to the restriction of their mechanical properties and the thermal stability. To combine both merits (the high-color-purity emission of the lanthanide compound containing the inorganic network and the flexible and transparent properties of the organic polymer), polymer-based rare earth hybrid materials have attracted plenty of interest14,34-37 as high-energy density batteries, sensors, and electrochromic and photoelectrochemical solid-state devices. The organic polymers, such as poly(methyl methacrylate),38 poly(vinyl pyridine),39 poly(vinylpyrrolidone),40,41 poly(ethylene glycol),42 and so on, are suitable to cooperate with lanthanide materials containing the Si-O network due to their low optical absorption in the ultraviolet-visible region, easily processed method, and low synthetic cost. The organic-inorganic polymeric hybrids provide good mechanical strength,43,44 high thermal stability,45 and excellent transparency for optical materials46,47 proved by the large value of ultimate strength and the rupture energy. Especially as a hard Lewis base the polymer

10.1021/jp9016807 CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

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poly(ethylene glycol) (PEG) could associate with hard acid rare earth ions.48 It is reported that the presence of PEG200 (poly(ethylene glycol) with the molecular weight of 200) in the containing Eu3+ complex doped into SiO2 gel has decreased the spectral width and increased the decay time as well as the emission intensities of Eu3+, preventing the quenching of the electronic transition.49 PEG400 (poly(ethylene glycol) with the molecular weight of 400) also has been introduced into the rare earth complex Eu(Phen)2Cl3 to investigate mechanical and luminescence properties.42 However, all these lanthanide hybrid materials have been connected with the organic polymers through the doping methods, for example, by hydrogen bonds. Therefore, in recent years our group has put forward a novel path to construct polymeric molecular-based lanthanide hybrids, especially containing β-diketone,50 where organic polymer and ligand could cooperate with the center ions through covalent bonds, such as coordinated bonds, silicon-oxygen bonds, and so on.51-54 In this paper, we have utilized two ligands (βdiketone and 1,10-phenanthroline) to complete the energy transfer system with Eu3+ and constructed the inorganic/organic networks between the ligands and the polymer PEG400 to obtain the hybrid material with high luminescent and thermal stabilities. 2. Experimental Section 2.1. Physical Measurement. FTIR spectra were measured within the 4000-400 cm-1 region by an infrared spectrophotometer with the KBr pellet technique. The ultraviolet absorption spectra (5 × 10-4 mol · L-1 chloroform solution) and the ultraviolet-visible diffuse reflection spectra of the powder samples were recorded by an Agilent 8453 spectrophotometer and a BWS003 spectrophotometer with the reference of BaSO4, respectively. Scanning electronic microstructure (SEM) was measured on Philip XL30. The fluorescence excitation and emission spectra were obtained by a RF-5301 spectrophotometer: excitation and emission slit width ) 3 nm. Luminescence lifetime data were carried out on an Edinburgh FLS920 phosphorimeter using a 450 W xenon lamp as the excitation source. All above measurements were completed under room temperature. Thermogravimetry (TG) data were obtained on Netzsch, model STA 409PC in the following conditions: atmosphere of nitrogen air, heating/cooling rate of 10 °C/min with Al2O3 crucible. 2.2. Chemicals. Europium nitrates were obtained by dissolving Eu2O3 in concentrated nitric acid (Eu3+ for the trivalent europium ion), and tetraethoxysilane (TEOS, Aldrich) was distilled and stored under a nitrogen atmosphere. The crosslinking agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) was purchased from the Lancaster company; the two ligands 2-thenoyltrifluoroacetone (TTA), 1,10-phenanthroline (Phen), and poly(ethylene glycol) (PEG400 for the polymer with the molecular weight of 380-430) were purchased from Shanghai Chemical Plant; and the solvent tetrahydrofuran (THF) was used after desiccation with anhydrous calcium chloride. N,N-Dimethyl formamide (DMF), ethanol, and hexane are analytically pure. 2.3. Synthetic Procedures. Description for the Acronyms of the Obtained Hybrid Materials (PEG-Si, TTA-Si, TTA-Si-Eu, TTA-Si-Eu-Phen, TTA-Si-Eu-PEG, and TTA-Si-Eu-Phen-PEG). PEG-Si delegates the network precursor formed by the reaction between poly(ethylene glycol) and 3-(triethoxysilyl)-propyl-isocyanate. TTA-Si delegates the organic cross-linking precursor formed by the reaction between 2-thenoyltrifluoroacetone and 3-(triethoxysilyl)-propyl-isocyanate. TTA-Si-Eu delegates the final binary hybrid formed by the coordination reaction between the organic cross-linking precursor TTA-Si and the europium ion and the hydrolysis

Qiao and Yan reaction between the organic cross-linking precursor TTA-Si and tetraethoxysilane TEOS, while TTA-Si-Eu-Phen delegates the ternary hybrid with the same hydrolysis process as TTA-Si-Eu, but a different coordination reaction between the organic cross-linking precursor TTA-Si, the terminal ligand 1,10-phenanthroline Phen, and the europium ion due to the addition of the terminal ligand 1,10-phenanthroline Phen. TTA-Si-Eu-PEG delegates the binary hybrid formed by the coordination reaction between the organic cross-linking precursor TTA-Si and the europium ion and the hydrolysis reaction between the organic precursor TTA-Si, the network precursor PEG-Si, and tetraethoxysilane TEOS, while TTA-Si-EuPhen-PEG delegates the ternary hybrid with the same hydrolysis process as TTA-Si-Eu-PEG, but a different coordination reaction between the organic precursor TTA-Si, the terminal ligand 1,10-phenanthroline Phen, and the europium ion due to the addition of the terminal ligand 1,10-phenanthroline Phen. 2.3.1. Synthesis of Polymer Precursors (PEG-Si). Poly(ethylene glycol) PEG400 (5 mmol, 2.150 g) was dissolved in 20 mL of dehydrated tetrahydrofuran (THF) completely under nitrogen atmosphere purging, and then 3-(triethoxysilyl)-propylisocyanate (TEPIC) with 10 mmol (2.475 g) was dropwise added into the solution. The mixture was heated and refluxed at 65 °C in a covered flask for approximately 10 h at the nitrogen atmosphere. The obtained materials were concentrated under room temperature to remove the solvent THF using a rotary vacuum evaporator, and the viscous liquid was obtained. Then the liquid sample was dissolved in absolute ethanol, and 20 mL of hexane was added into the solution to purify the liquid sample. Subsequently, the solution was washed and extracted. The above procedures involving dissolution and extraction were repeated three times. At last the pure colorless viscous liquid was obtained and preserved in a vacuum with the yield of 85% (see Figure 1a). PEG-Si (C38H80O18Si2N2) 1HNMR data: δ0.68 (4H, t), δ1.25 (18H, t), δ1.64 (4H, t), δ3.18 (4H, m), δ3.54 (12H, m), δ3.77 (20H, t), δ3.83 (4H, t), δ3.91 (4H, t), δ4.08 (4H, t), δ4.39 (4H, t), δ7.30 (2H, t). From 1HNMR data of the network precursor PEG-Si, it is proved that the polymerization of the PEG400 is about 8-9, and the 3-(triethoxysilyl)-propyl-isocyanate (TEPIC) has already grafted onto the polymer PEG400. 2.3.2. Synthesis of the Cross-Linking Precursor Containing Si-O Chemical Bonds (TTA-Si).53 2-Thenoyltrifluoroacetone (TTA) (2 mmol, 0.444 g) was first dissolved in 20 mL of dehydrate tetrahydrofuran (THF), and NaH (4 mmol, 0.096 g) was added into the solution with stirring at the temperature of 65 °C. Two hours later, 4 mmol (0.990 g) of 3-(triethoxysilyl)propyl-isocyanate (TEPIC) was dropwise added into the refluxing solution. The mixture was heated at 65 °C in a covered flask for approximately 12 h at the nitrogen atmosphere. The obtained material was concentrated under room temperature to remove the solvent THF using a rotary vacuum evaporator, and the brown liquid was obtained. Then the liquid sample was purified by the same procedures as above, and the pure brown liquid was preserved in a vacuum with the yield 75% (see Figure 1b). TTA-Si (C28H47O10F3N2Si2S) 1HNMR data: δ0.64 (4H, t), δ1.25 (18H, t), δ1.59 (4H, t), δ3.11 (4H, t), δ3.83 (12H, m), δ5.99 (2H, t), δ6.96 (1H, d), δ7.34 (1H, d), δ7.52 (1H, d). From 1 H NMR data of the precursor TTA-Si, it is proved that the 3-(triethoxysilyl)-propyl-isocyanate (TEPIC) has grafted onto the ligand TTA.

Lanthanide/TTA/Si-O Network/PEG Hybrid Materials

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Figure 1. Scheme of synthesis processes of the precursors PEG-Si and TTA-Si.

2.3.3. Synthesis of the Hybrid Material Bonding with Si-O Networks and Organic Carbon Chains (TTA-Si-Eu, TTA-SiEu-PEG,TTA-Si-Eu-Phen,andTTA-Si-Eu-Phen-PEG). TTA-Si-Eu and TTA-Si-Eu-PEG: The organic crosslinking precursor TTA-Si was dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Eu(NO3)3 · 6H2O (0.7 mmol, 0.317 g) was dropwise added into the solution while stirring. After 6 h, a stoichiometric amount of TEOS and H2O were added into the mixed solution (while for TTA-Si-Eu-PEG a stoichiometric amount of the precursor PEG-Si, TEOS, and H2O were added into the mixed solution) after the coordination reaction had completed between the organic cross-linking precursor TTA-Si and the europium ion, which accompanied the addition of one drop of dilute hydrochloric acid to promote hydrolysis. The molar ratio of RE(NO3)3 · 6H2O:TTA-Si:TEOS:H2O was 1:3:6:24 (TEOS 4 mmol, 0.833 g, and H2O 0.288 g) (while for TTA-Si-Eu-PEG, the molar ratio of RE(NO3)3 · 6H2O:TTA-Si:PEG-Si:TEOS: H2O was 1:3:3:3:24). After the treatment of hydrolysis, an appropriate amount of hexamethylenetetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically to achieve a single phase in a covered Teflon beaker, and then it was aged at 70 °C until the onset of gelation in about 5 days. The gels TTA-Si-Eu and TTA-Si-Eu-PEG were collected as monolithic bulks and were ground into powdered samples for the photophysical studies. TTA-Si-Eu-Phen and TTA-Si-Eu-Phen-PEG: The precursor TTA-Si was dissolved in N,N-dimethyl formamide (DMF) solvent, and a stoichiometric amount of Eu(NO3)3 · 6H2O (0.7 mmol, 0.317 g) was dropwise added into the solution while stirring. After 2 h, the terminal ligand 1,10-phenanthroline (Phen) (0.7 mmol, 0.108 g) was added into the mixture, and then another 6 h later, a stoichiometric amount of TEOS and H2O were added into the mixed solution (while for TTA-EuPhen-PEG a stoichiometric amount of the precursor PEG-Si, TEOS, and H2O were added into the mixed solution) when the coordination reaction had completed between precursors and europium ions, which accompanied the addition of one drop of dilute hydrochloric acid to promote hydrolysis. The molar ratio of RE(NO3)3 · 6H2O:TTA-Si:Phen:TEOS:H2O was 1:3:1:6:24 (TEOS 4 mmol, 0.833 g, and H2O 0.288 g) (while for

TTA-Eu-Phen-PEG, the molar ratio of RE(NO3)3 · 6H2O: TTA-Si:Phen:PEG-Si:TEOS:H2O was 1:3:1:3:3:24). After the treatment of hydrolysis, an appropriate amount of hexamethylenetetramine was added to adjust the pH to 6-7. The mixture was agitated magnetically to achieve a single phase in a covered Teflon beaker, and then it was aged at 70 °C until the onset of gelation in about 6 days. The gels TTA-Si-Eu-Phen and TTA-Si-Eu-Phen-PEG were collected as monolithic bulks and were ground into powdered samples for the photophysical studies (seen from Figure 2). 3. Results and Discussion 3.1. Ultraviolet (Visible Diffuse Reflection) and Infrared Spectra Analysis for the Raw Materials and Polymeric Hybrids. 3.1.1. UltraWiolet and UltraWiolet-Visible Diffuse Reflection Absorption Spectra. Figure 3 exhibits ultraviolet absorption spectra of (A) TTA, (B) TTA-Si, (C) TTA-Si-Eu, (D) TTA-Si-Eu-PEG, (E) TTA-Si-Eu-Phen, and (F) TTA-Si-Eu-Phen-PEG. It is observed that a blue shift (AfB) of the major π-π* electronic transitions (from 325 to 268 nm) occurred, which has indicated that the electron distribution of the conjugated system has changed when the TEPIC has been introduced into the β-diketone ligand TTA. The procedure to synthesize the organic cross-linking precursor TTA-Si has decreased the wavelength of the TTA-Si up to 57 nm, and the energy difference of the electronic orbits has increased to lead to the less conjugated properties, causing electron transition to be harder to realize after the grafted procedure. The less conjugated properties of the TTA-Si attribute to the addition of the TEPIC since the complicated configuration in a relatively small space restricts the electronic planar transition. Compared to line B, lines C and D have two broad peaks at 272 and 341 nm for C and at 268 and 339 nm for D, respectively, very different from line A, and the changes on the peak shapes have indicated that the organic cross-linking precursor TTA-Si has already coordinated to Eu3+ to form the polymeric hybrid, which has a great deal of influence on the electron distribution of the conjugated system. Moreover, the lines C and D have shown a similar shape with exiguous difference of the shift of about 4 nm, since the hybrid TTA-Si-Eu-PEG contained the polymer PEG, but the PEG

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Figure 2. Scheme of synthesis processes of hybrid polymeric materials TTA-Si-Eu-Phen and TTA-Si-Eu-Phen-PEG.

did not make any contribution to the coordinating procedure with Eu3+ although it has played an important role in the hydrolysis and copolycondensation with the tetraethoxysilane (TEOS) via the sol-gel process. Therefore, the addition of the polymer PEG has not influenced the shapes of the peaks. However, the small blue shift of about 10 nm (CfE, DfF) from 339 to 329 nm has appeared, and the shapes of the peaks have changed slightly, the height of the first peak being lower, when the terminal ligand Phen has been introduced into the hybrid since the terminal ligand Phen has participated to coordinate to Eu3+ resulting in the changes of the electron distribution in the conjugated system of the hybrid materials. Furthermore, the shapes of the lines E and F have were similar, as the only difference between them lies in the polymer PEG, which has little influence on the electron distribution, whose major influence on the microstructure is discussed later in the paper. The ultraviolet-visible diffuse reflection absorption data of the raw material (the ligand TTA) and hybrid materials are given in Figure 4 (A for TTA-Si-Eu, B for TTA-Si-Eu-PEG, C for TTA-Si-Eu-Phen, and D for TTA-Si-Eu-Phen-PEG).

Figure 3. Ultraviolet absorption spectra of the free ligand TTA (A), the precursor TTA-Si (B), and the hybrid polymeric materials TTA-Si-Eu (C), TTA-Si-Eu-PEG (D), TTA-Si-Eu-Phen (E), and TTA-Si-Eu-Phen-PEG (F).


Figure 4. Ultraviolet-visible diffuse reflection absorption spectra of the ligand TTA and the hybrid polymeric materials TTA-Si-Eu (A), TTA-Si-Eu-PEG (B), TTA-Si-Eu-Phen (C), and TTA-SiEu-Phen-PEG (D).

Figure 5. Fourier transform infrared spectra of the ligand TTA, the polymer PEG400, the precursors TTA-Si and PEG-Si, and the hybrid materials TTA-Si-Eu (A), TTA-Si-Eu-PEG (B), TTA-Si-Eu-Phen (C), and TTA-Si-Eu-Phen-PEG (D).

It is observed that the wide absorption bands of A, B, C, and D are located at about 240-390 nm, which partially overlaps with the absorption bands in the fluorescent excitation spectra (wide bands at 320-400 nm in Figure 8a), and are simultaneously located in the absorption extent of the free ligand TTA. It is illustrated that the four kinds of hybrid materials absorb energy within a similar ultraviolet-visible extent, which has derived from the ligand TTA, not due to the polymer PEG or the terminal ligand Phen, since even without the PEG and Phen the hybrid still has absorption in the corresponding extent.


Figure 6. Thermogravimetry trace (TG) and differential thermogravimetry trace (DTG) of the hybrid materials TTA-Si-Eu (A) and TTA-Si-Eu-Phen-PEG (B).

Moreover, the most obvious reverse sharp peaks of the lines A, B, C, and D are all located at 609 and 702 nm, which assign to the characteristic emission peaks of the Eu ion. The above result indicates that TTA has absorbed energy in the ultravioletvisible region to transfer the energy to the corresponding hybrid materials through the intramolecular energy transfer system. 3.1.2. Fourier Transform Infrared Spectra. The FTIR spectra of the free β-diketone ligand TTA, the polymer PEG400, and the precursors TTA-Si and PEG-Si are shown in Figure 5a. From A to B, it could be observed that the stretching vibrations of -CH2- at 3113 cm-1 in TTA took place by a strong triple broadband located at 2889, 2930, and 2977 cm-1 (B), which originated from the three methylene groups of 3-(triethoxysilyl)-propyl isocyanate (TEPIC). In the spectra of PEG, the broad peak at 2882 cm-1 derived from the eight or nine associated methylene groups which have been proved by 1 H NMR data has shifted to 2916 cm-1 in PEG-Si and is also influenced by the methylene groups of 3-(triethoxysilyl)-propyl isocyanate (TEPIC). New peaks at 1519 and 1621 cm-1 have emerged in TTA-Si assigned to the stretching vibration of CdO and the bending vibration of -NH- in the group -CONH-, 1533 and 1648 cm-1 for PEG-Si. Moreover, the peaks at 1072 and 1167 cm-1 in TTA-Si have indicated the absorption of the stretching vibrations of the groups of C-Si and Si-O, while they are at 1079 and 1255 cm-1 for PEG-Si. The above spectral data have provided the evidence that TEPIC has been grafted onto the ligand TTA and the polymer PEG successfully, respectively. In Figure 5b, the lines of A, B, C, and D represent the polymeric hybrid materials TTA-Si-Eu, TTA-SiEu-PEG, TTA-Si-Eu-Phen, and TTA-Si-Eu-Phen-PEG, respectively. It is shown that the formation of the Si-O-Si framework is evidenced by the broad bands located at about 1055-1137, 790, and 460 cm-1, which is attributed to the asymmetric stretching vibration, symmetric stretching vibration, and planar bending vibration of the group Si-O-Si, respectively, indicating the success of hydrolysis and copolycondensation procedures. Ulteriorly, the sharp peaks located at 1377 and 1540 cm-1 in A, B, C, and D have illustrated the existence of the nitrate group. 3.2. Thermal Characteristics and Microstructure Analysis for Polymeric Hybrids. 3.2.1. Thermal Stability Analysis. Figure 6 shows the thermogravimetry trace (TG) (A and B) and differential trace (DTG) (C and D) of the hybrid polymeric materials TTA-Si-Eu (black lines) and TTA-Si-EuPhen-PEG (red lines). Seen from the TG curve A, the hybrid material TTA-Si-Eu has lost the mass (about 12%) from 131 °C until 258 °C with the largest speed when the temperature

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Figure 7. SEM images of the hybrid materials TTA-Si-Eu (A), TTA-Si-Eu-PEG (B), TTA-Si-Eu-Phen (C), and TTA-Si-Eu-Phen-PEG (D).


Figure 8. Excitation (a) and emission spectra (b) of the hybrid materials TTA-Si-Eu (A), TTA-Si-Eu-PEG (B), TTA-Si-Eu-Phen (C), and TTA-Si-Eu-Phen-PEG (D).

has climbed to 235 °C. It is deduced that the residual solvent DMF has decomposed, whose boiling point is at about 170 °C, so part of the hydrogen and nitrogen atoms have formed the gas with a few oxygen atoms of the hybrid material. The line B has also shown a mass loss of about 16% of the solvent from 127 to 258 °C with the largest speed at 237 °C. Moreover, according to the molecular structure and weight of the hybrid material TTA-Si-Eu, the ligand TTA occupies about 14% and 22.6% for TEPIC, while in the hybrid material TTA-Si-EuPhen-PEG, the ligand TTA and Phen together occupy 13% and 29% for TEPIC. The graph shows us a mass loss of about 25%, and when the temperature is 313 °C, the material has lost the weight most quickly in A, which is attributed to the decomposition of all the ligand TTA and part of the carbon atoms of the TEPIC. For the hybrid material TTA-Si-EuPhen-PEG, the line B shows a mass loss of about 23% with the largest speed at 308 °C, also indicating all the ligands TTA and Phen have been decomposed and a small part of the carbon atoms of the TEPIC have fallen off. In addition, the hybrid material TTA-Si-Eu-Phen-PEG has lost a mass of about 14% with the sharpest peak at 406 °C seen from the line D, which is not observed in the line C. Therefore, it is speculated that that mass loss has been contributed to by the decomposition of the polymer PEG. Ulteriorly, the hybrid material TTA-Si-Eu retains the weight of 53%, according to the actual proportion of silicon, oxygen atoms, and rare earth ions (58%) calculated in terms of the molecular formula. The hybrid material TTA-Si-Eu-Phen-PEG retains the weight of 32%, a little less than the actual proportion of silicon, oxygen atoms, and rare earth ions of 45% in the molecular formula. The possible presumption is that the addition of the polymer derivative

J. Phys. Chem. B, Vol. 113, No. 35, 2009 11871 PEG-Si has reacted with TEOS and the organic cross-linking precursor TTA-Si to form the Si-O network through the hydrolysis and copolycondensation process, so the decomposition of PEG has affected the configuration stability of the silicon-oxygen networks. 3.2.2. Scanning Electron Micrographs. The scanning electron micrographs of the hybrid polymeric materials demonstrate that the homogeneous, molecular-based materials were obtained with covalent bonds between the organic ligand β-diketone or polymer PEG and the inorganic matrix and the coordinate bonds between the two kinds of ligands and rare earth ions, constructing a huge complicated molecular system (Figure 7) (A for TTA-Si-Eu, B for TTA-Si-Eu-PEG, C for TTA-SiEu-Phen, and D for TTA-Si-Eu-Phen-PEG). Compared with the hybrid material with doped lanthanide complexes, which generally experience the phase separation phenomena,55,56 in the polymeric hybrid materials we obtained in this paper the phase separation phenomena do not exist, and inorganic and organic phases integrate their distinct properties together. The ligand TTA (2-thenoyltrifluoroacetone) belongs to the multidimensional β-diketone with the thiophene ring, two carbonyl groups, and a trifluoromethyl group, so its derivative TTA-Si may readily form a two-dimensional layer-like or three-dimensional network-like microstructure, and its corresponding complex still keeps this trend. The hybrid A (TTA-Si-Eu) is composed by many regular and uniform spheres with the same diameter size of about 2 µm. The hybrid B (TTA-Si-Eu-PEG) is also composed by many particles with the diameter of about 2 µm, which is the same with A. However, in the overall view, the microstructure B has shown less orderly arrangement, and the particles have not grown into the regular sphere. We have speculated that besides the growth trend of derivative TTA-Si there exists another growth trend to form Si-O networks in the hybrid materials, and this trend has been enhanced due to the addition of the polymeric derivative PEG-Si. The hybrid A (TTA-Si-Eu) merely contains the derivative TTA-Si, whose major trend to form the sphere microstructure has overcome the minor trend derived from the polymeric derivative PEG-Si to form Si-O networks, while besides TTA-Si the hybrid B (TTA-Si-Eu-PEG) also contains the polymer derivative PEG-Si, which has held the original properties of the polymer and has contributed to form Si-O networks with the TTA-Si through the hydrolysis and copolycondensation process. So, the two trends have made their own efforts to compose the final microstructure of the hybrid (TTA-Si-Eu-PEG). The hybrid C (TTA-Si-Eu-Phen) is also composed by many homogeneous spheres with the same diameter size of about 5 µm, as a result of the main trend of derivative TTA-Si to easily form the two-dimensional layer or three-dimensional network polymeric microstructures. However, the ligand Phen has participated to coordinate to the rare earth ions, so the hybrid C (TTA-Si-Eu-Phen) has the larger configuration in the wider space compared to the hybrid A (TTA-Si-Eu), which accelerates the sphere to grow to form the final microstructure with a larger size than the hybrid A. The hybrid D (TTA-Si-Eu-Phen-PEG) has shown many particles, which have not grown into spheres as the hybrid B TTA-Si-Eu-PEG, and there even exist many uniform hiberarchy and dendritic stripes on the surface (see Figure 7 (D1)). ItisilluminatedthatcomparedtothehybridB(TTA-Si-Eu-Phen) the addition of the polymer PEG has a significant influence on the hydrolysis and copolycondensation process since the PEG possesses long carbon chains on the vertical level and brings the expansive extensity, which is propitious to the growth of

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the polymeric stripelike structure of Si-O-Si due to the decrease of the steric hindrance effect. Therefore, the growth trend to construct the polymeric network with stripes has become the major trend, overcoming the trend of the derivative TTA-Si to form the three-dimensional spheres. Compared to the hybrid B (TTA-Si-Eu-PEG) in the hybrid D, the addition of the terminal ligand Phen has changed the conjugated electronic system, spatial configuration, and coordination bond and disturbed the major growth trend of the derivative TTA-Si. Therefore, the hybrid D has grown according to the primary growth trend to form the final trunklike microstructure. Furthermore, it is indicated that many factors control the microstructure of the hybrids, such as the coordination number, spatial configuration, conjugated electronic system, substance ingredients, and so on. 3.3. Photoluminescent Properties of Polymeric Hybrids. 3.3.1. Fluorescent Characteristic Emission Analysis. Figure 8 shows the excitation (a) and emission spectra (b) of hybrid polymeric materials containing Eu3+ covalently bonded into silicon-oxygen networks and carbon chains (A for TTA-Si-Eu, B for TTA-Si-Eu-PEG, C for TTA-Si-Eu-Phen, and D for TTA-Si-Eu-Phen-PEG). The excitation spectra of these materials were all obtained by monitoring the characteristic emission wavelength of Eu3+ at 613 nm. For the excitation spectra, the broad absorption peaks are at about 380 nm, suggesting the effective absorption of the Eu-TTA or Eu-TTA-Phen system. As a result, in Figure 8b the characteristic emissions of the hybrid materials have presented the transitions of 5D0f7FJ (J ) 0, 1, 2, 3, 4) at about 577, 590, 610, 650, and 702 nm for Eu3+. It is seen from all the lines that the emission background of ligand TTA completely vanished under the excitation of 380 nm, and only the characteristic emissions of Eu3+ can be found without the high baseline of the Si-O network matrix, indicating that the ligand TTA has completely transferred it to the center metal ions (Eu3+). Among these emission peaks of the hybrid materials, red emission intensities (arbitrary unit, a.u.) of the electric dipole transition of 5D0f7F2 at about 610 nm are all larger than the orange emission intensities of the magnetic dipole transition of 5D0f7F1 at about 590 nm. Since the 5D0f7F2 transition strongly varies with the local symmetry environment of Eu3+, while the 5 D0f7F1 transition is independent of that, the emission spectra indicate that the Eu3+ is situated in an environment without inversion symmetry.57 We have always expected that through this efficient transfer mechanism leaching of the photoactive molecules and clustering of the emitting centers could be avoided, and higher concentration of center metal ions will be realized. Other factors cannot still be excluded such as the relatively rigid structure of silica gel which limits the vibration of the hydroxyl group and prohibits nonradiative transitions. Furthermore, the nonsymmetry and the intensities of the electric dipole transition 5D0f7F2 will increase as the interaction of the rare-earth complex with its local chemical environment has been completed more entirely, so the integration ratio (I02/I01) of the 5 D0f7F2/5D0f7F1 transition has been widely used as an indicator of Eu3+ site symmetry. Ulteriorly, in Figure 8b the shapes of the emission peaks of A and B show some resemblance, and the intensities of B are obviously higher than those of A with the relative intensities ration (I02/I01) of 5D0-7F2/5D0-7F1 transition in A of 7.2, 10.6 in B. However, the shapes of emission peaks located at 589 and 610 nm in C and D are different from those in A and B, and all the peaks in C have shown almost the same relative intensities with those in D, which are much higher than the intensities of peaks in A and

Qiao and Yan B. In addition, the relative intensities ratios of 5D0-7F2/5D0-7F1 transition are also similar to each other, 7.4 in C and 7.0 in D. These data reflect the actual coordination environment of the Eu3+ and interpret the predicted structure in Figure 2. We have deduced that there are two reasonable factors. One is that neither of the hybrids TTA-Si-Eu nor TTA-Si-Eu-PEG contains the terminal ligand Phen, which has a crucial influence on the relative intensities. It is concluded from many experiments and researches that the most important factor influencing the luminescence properties of rare earth complexes is the intramolecular energy transfer efficiency, which mainly depends on the two energy transfer processes.58-60 One is from the lowest triplet level of the ligand to the emissive energy level of the center ion (Ln3+),61,62 and the other is the reverse energy transition by the thermal deactivation mechanism. The energy transfer rate constant (kT) is dependent on the energy difference (∆ETr-Ln3+) between the lowest triplet level energy of the ligand and the resonant emissive energy of the central ion. On the basis of the above two factors, the conclusion can be drawn that ∆E(Tr-Ln3+) can have ambilateral influence on both energy transfer processes, and there should exist an optimal energy difference.63 Moreover, since the lowest triple state energy level of TTA ligand is 20 400 cm-1, 22 173 cm-1 for Phen, and the resonance energy level of Eu3+ (5D1) is 19 020 cm-1,57,64,65 it can be predicted that the ligands TTA, Phen, and Eu3+ could complete the intramolecular energy transfer efficiently. Compared to the hybrid TTA-Si-Eu, the addition of the polymer PEG into TTA-Si-Eu has increased the relative intensities since the PEG has played an important role in the hydrolysis and copolycondensation processes with the tetraethoxysilane (TEOS) instead of the water molecules to avoid the luminescence quenching due to the vibration of the hydroxyl groups and has also caused the frameworks of the matrices to be organized more regularly. In addition, the polymer PEG has possibly replaced a small part of coordination water molecules around Eu3+ so that the relative intensities ration (I02/I01) has change from 7.2 in A to 10.6 in B, slightly influencing the coordinating environment around Eu3+. When the terminal ligand Phen is added, it absorbs the energy and transfers the energy to the first ligand TTA, then to Eu3+. The terminal ligand Phen has usually been used as the terminal ligand in the coordination process and sensitized the center ions with other ligands together due to its weak coordination ability and the replacement of water molecules.66-68 Although PEG could replace the water molecules in the hydrolysis process to avoid the quenching effect to facilitate the emission, its influence has been restricted by the environment. Since the water molecules coordinating to Eu ions were replace by the terminal ligand Phen and the energy transfer system has been completed adequately in the system of C (TTA-Si-Eu-Phen), the influence of PEG could be ignored compared to efficient energy transfer completed in TTA-SiEu-Phen. Therefore, there is no obvious change in D TTA-SiEu-Phen-PEG after the addition of PEG, and the relative intensities ration (I02/I01) has not obviously changed from C to D. However, the energy transfer system in the system of TTA-Si-Eu has not been consummated as that of TTA-SiEu-Phen, and the addition of PEG has shown some advantages on the luminescent emission, so the relative intensities of TTASi-Eu-PEG are higher that those of TTA-Si-Eu. Seen from C TTA-Si-Eu-Phen and D TTA-Si-Eu-Phen-PEG, the highest peaks at 611 nm have split, indicating Eu3+ is located in two kinds of coordination environments due to the existence of the two kinds of ligands (TTA and Phen).



3.3.2. Luminescence Decay Times (τ) and Emission Quantum Efficiency (η). According to the emission spectra and the lifetime of the Eu3+ first excited level (τ, 5D0), the emission quantum efficiency (η) of the 5D0 excited state can be determined. Assuming that only nonradiative and radiative processes are essentially involved in the depopulation of the 5D0 excited state, η can be obtained through the radiative transition rate (Ar) divided by the total transition rates69 (the sum of the radiative Ar and nonradiative Anr transition rates). The radiative transition rate (Ar) could be obtained by summing over the radiative rates A0J for each 5D0f7FJ (J ) 0-4) transition of Eu3+. The branching ratio for the 5D0f7F5,6 transitions can be neglected as they are not detected experimentally, whose influence can be ignored in the depopulation of the 5D0 excited state. Since magnetic dipole 5D0f7F1 is practically independent of the chemical environments around Eu3+ and considered as an internal reference, the experimental coefficients of spontaneous emission A0J can be calculated according to the equation70-72 as follows

A0J ) A01(I0J /I01)(ν01 /ν0J)

(1)

A01 is the Einstein coefficient of spontaneous emission between the 5D0 and 7F1 energy levels. In vacuum, A01 could be used as a value of 14.65 s-1, while in the air atmosphere, the value of A01 can be determined to be 50 s-1 approximately (A01 ) n3A01(vac)),73 when an average index of refraction n equal to 1.506 was considered. I01 and I0J are the integrated intensities of the 5 D0 f 7F1 and 5D0 f 7FJ transitions (J ) 0-4), and ν0J, referred to the energy barycenter, can be determined as the reciprocal of the wavelength where the emission peaks of Eu3+’s 5D0 f 7 FJ transitions are located. The emission intensity, I, could be taken as integrated intensity S of the 5D0f7FJ emission curves. On the basis of refs 66, 68, 74, and 75 and the lifetime (τ), radiative (Ar) and nonradiative (Anr) transition rates are related through the following equation

Atot ) 1/τ ) Ar + Anr

(2)

On the basis of the above discussion, the quantum efficiencies of the four kinds of hybrid polymeric materials can be determined as shown in Table 1. The value η mainly depends on the values of two factors: one is lifetime and the other is integrated intensity ratio of I02/I01. As can be clearly seen from Table 1, the quantum efficiencies of TTA-Si-Eu-PEG are much higher than that of TTA-Si-Eu and those of TTA-SiEu-Phen-PEG are slightly higher than those of TTA-Si-EuPhen, indicating that the addition of the polymer PEG-Si, which interacted with TEOS in a hydrolysis and copolycondensation process, has restricted hydroxyl group vibrations to improve the quantum efficiencies. However, since the terminal ligand Phen has three aromatic rings and possesses a larger spatial configuration, the addition of the terminal ligand Phen has not enhanced the quantum efficiency obviously in TTA-Si-EuPhen compared with TTA-Si-Eu, possibly due to the steric hindrance effect and the change on energy transfer system. The hybrids TTA-Si-Eu and TTA-Si-Eu-Phen have a large nonradiative transition rate seen from Table 1, also proving the quenching effect arousal by the hydroxyl group vibration. However, compared to TTA-Si-Eu-PEG, the fluorescence lifetime of TTA-Si-Eu-PEG is large while the relative intensities ratio of I02/I01 is smaller, so the quantum efficiency of TTA-Si-Eu-Phen-PEG has decreased, indicating that the hybrid polymer TTA-Si-Eu-PEG has the more effective red emission and the higher color purity. 3.3.3. Experimental Intensity Parameters (Ω). To investigate the possible structural changes around the center ion among these hybrid materials, the experimental intensity parameters Ω should be calculated from the emission spectra according to the certain method.72,76 The spontaneous emission probability A of the transition is related to its dipole strength according to the equation73,76-78

TABLE 1: Luminescence Efficiencies and Lifetimes of the Solid Hybrid Materials

a

hybrid systems

TTA-Si-Eu

TTA-Si-Eu-PEG

TTA-Si-Eu-Phen-PEG

TTA-Si-Eu-Phen-PEG

V00 (cm-1)a V01 (cm-1)a V02 (cm-1)a V03 (cm-1)a V04 (cm-1)a I00b I01b I02b I03b I04b A00 (s-1) A01 (s-1) A02 (s-1) A03 (s-1) A04 (s-1) τ (ms)c Arad (s-1) τexp-1 (s-1) Anrad (s-1) I02/I01 η (%) Ω2 (10-20 cm2) Ω4 (10-20 cm2)

17331 16949 16367 15408 14347 56.1 210.7 1526.2 31.3 20.1 13.0 50.0 375.1 8.2 5.6 0.564 451.9 1773.0 1321.1 7.2 25 10.87 0.37

17331 16949 16367 15385 14306 95.4 463.0 4930.3 62.6 30.4 10.1 50.0 551.4 7.4 3.9 0.724 622.8 1381.2 758.4 10.6 45 15.98 0.26

17301 17007 16393 15361 14245 98.5 751.9 5559.9 81.1 41.4 6.4 50.0 383.6 6.0 3.3 0.554 449.3 1805.1 1355.8 7.4 25 11.12 0.22

17301 17007 16393 15361 14245 114.6 834.6 5852.0 88.4 46.4 6.7 50.0 363.7 5.9 3.3 0.731 429.7 1368.0 938.3 7.0 31 10.54 0.22

The energies of the 5D0 f 7FJ transitions (υ0J). b The integrated intensity of the 5D0 f 7FJ emission curves. c For the 5D0 f 7F2 transition of Eu3+.

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Qiao and Yan

A ) (64π4υ3)/[3h(2J + 1)]{[(n2 + 2)2 /9n]S(ED) + 2

n S(MD)} (3) υ is the average transition energy in cm-1; h is Planck’s constant; and 2J + 1 is the degeneracy of the initial state (1 for 5D0). S(ED) and S(MD) are the electric and magnetic dipole strengths, respectively. The factors containing the medium’s refractive index n result from local field corrections that convert the external electromagnetic field into an effective field at the location of the active center in the dielectric medium. All the transitions from 5D0 to 7F0,3,5 (J ) 0, 3, 5) are forbidden both in magnetic and induced electric dipole schemes (S(ED) and S(MD) are zero). The transition from 5D0 to 7F1 (J ) 1) is the isolated magnetic dipole transition and has no electric dipole contribution, which is practically independent of the center ion’s chemical environment and can be used as a reference as mentioned above. Besides, the 5D0f7F6 transition could not be experimentally detected, and it is not necessary to determine its experimental intensity parameter. So we only need to estimate the two parameters (Ω2, Ω4) related to the two purely induced electric dipole transitions 5D0f7F2,4 on the basis of only three parameters Ωλ using eq 472,76-78

A ) (64e2π4υ3)/[3h(2J + 1)][(n2 + 2)2 /9n] ×

∑ Ωλ|< 7FJ||U(λ)||5D0>|2

(4)

e is the electronic charge. With the refraction index n ) 1.506 and |2, values are the square reduced matrix elements whose values are 0.0032 and 0.0023 for J ) 2 and 4, respectively. The Ω2 and Ω4 intensity parameters for the four hybrid materials are shown in Table 1. The obvious change on Ω2 between the hybrid materials TTA-Si-Eu and TTA-SiEu-PEG has shown that the structural environment around the Eu ion varied due to the hypersensitivity of the 5D0f7F2 transition. The larger Ω2 in TTA-Si-Eu-PEG has indicated that the symmetry of the Eu3+ site has been disturbed due to the addition of PEG possibly to replace the water molecules coordinating to the Eu ion. While the difference on Ω2 between the hybrid materials TTA-Si-Eu-Phen and TTA-Si-EuPhen-PEG is not clear, the terminal ligand Phen has already replaced the coordination water molecules, so the addition of PEG has hardly contributed to the coordination structural environment around the Eu ion. 4. Conclusion In summary, we have developed a representative method for assembling the luminescent rare-earth molecular-based polymeric β-diketone hybrid materials with chemical bonds, which contain the Si-O network constructed by the organic polymer PEG400, TEOS, and the modified ligand TTA-Si through the sol-gel process. The precursors TTA-Si and PEG-Si were synthesized grafting 3-(triethoxysilyl)-propyl isocyanate onto TTA and PEG. Then the hybrid polymer was obtained after the coordination, hydrolysis, and condensation processes. The results reveal that the hybrid containing PEG-Si shows the less regular microstructure but relatively intense luminescent property and better quantum efficiency than the hybrid without PEG-Si. Furthermore, the photoluminescence spectra of the hybrids containing the terminal ligand Phen show the more effective intramolecular energy transfer mechanism than the hybrids without Phen. The thermal analysis results also prove

that the ternary hybrid containing PEG-Si and Phen shows thermal stabilities similar to the binary hybrid without Phen and PEG. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20671072) and Program for New Century Excellent Talents in University (NCET-080398). References and Notes (1) Sabbatini, N.; Guardigli, M.; Lehn, J. M. Coord. Chem. ReV. 1993, 123, 201. (2) Parker, D.; Williams, J. A. G. J. Chem. Soc., Dalton Trans. 1996, 3613. (3) Elbanowski, M.; Makowska, B. J. Photochem. Photobiol. A: Chem. 1996, 99, 85. (4) Richardson, F. S. Chem. ReV. 1982, 82, 541. (5) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (6) Li, H. H.; Inoue, S.; Machida, K.; Adachi, G. Chem. Mater. 1999, 11, 3171. (7) Whittakker, B. Nature 1970, 288, 157. (8) Weissman, S. I. J. Chem. Phys. 1942, 10, 214. (9) Alpha, B.; Ballardini, R.; Balzani, V.; Lehn, J. M.; Perathoner, S.; Sabbatini, N. Angew. Chem., Int. Ed. Engl. 1987, 26, 299. (10) Matthews, L. R.; Kobbe, E. T. Chem. Mater. 1993, 5, 1697. (11) Costa, V. C.; Vasconcelos, W. L.; Bray, K. L. J. Sol-Gel Sci. Technol. 1998, 13, 605. (12) Sokolnicki, J.; Maruszewski, K.; Strek, W.; Legendziewicz, J. J. Sol-Gel Sci. Technol. 1998, 13, 611. (13) Jin, T.; Inoue, S.; Tsutsumi, S.; Machida, K.; Adachi, G. J. NonCryst. Solids 1998, 223, 123. (14) Bekiari, V.; Pistolis, G.; Lianos, P. Chem. Mater. 1999, 11, 3189. (15) McGehee, M. D.; Bergstedt, T.; Zhang, C.; Saab, A. P.; O’Regan, M. B.; Bazan, G. C.; Srdanov, V. I.; Heeger, A. J. AdV. Mater. 1999, 17, 1349. (16) Yang, C. Y.; Srdanov, V.; Robinson, M. R.; Bazan, G. C.; Heeger, A. C. AdV. Mater. 2002, 14, 980. (17) Wolff, N. E.; Pressley, R. J. Appl. Phys. Lett. 1963, 8, 152. (18) Binnemans, K.; Go¨rller-Walrand, C. Chem. ReV. 2002, 102, 2303. (19) Binnemans, K.; Moors, D. J. Mater. Chem. 2002, 12, 3374. (20) Van Deun, R.; Moors, D.; De Fre´, B.; Binnemans, K. J. Mater. Chem. 2003, 13, 1520. (21) Sanchez, C.; Ribot, F. New. J. Chem. 1994, 18, 1007. (22) Li, H. R.; Lin, J.; Zhang, H. J.; Fu, L. S. Chem. Mater. 2002, 14, 3651. (23) Wang, Q. M.; Yan, B. Inorg. Chem. Commun. 2004, 7, 747. (24) Dong, D. W.; Jiang, S. C.; Men, Y. F.; Ji, X. L.; Jiang, B. Z. AdV. Mater. 2000, 12, 646. (25) Franville, A. C.; Zambon, D.; Mahiou, R. Chem. Mater. 2000, 12, 428. (26) Wang, Q. M.; Yan, B. J. Mater. Chem. 2004, 14, 2450. (27) Wang, Q. M.; Yan, B. J. Mater. Res. 2005, 20, 592. (28) Wang, Q. M.; Yan, B. Cryst. Growth Des. 2005, 5, 497. (29) Li, H. R.; Fu, L. S.; Zhang, H. J. Thin Solid Films 2002, 416, 197. (30) Carlos, L. D.; Ferreira, R. A. S.; Pereira, R. N.; Assuncao, M.; Bermudez, V. D. J. Phys. Chem. B 2004, 108, 14924. (31) Carlos, L. D.; Ferreira, R. A. S.; Rainho, J. P.; Bermudez, V. D. AdV. Funct. Mater. 2002, 12, 819. (32) Tanaka, H.; Takahashi, J.; Tsuchiya, J. J. Non-Cryst. Solids 1989, 109, 164. (33) Kubayashi, Y.; Kurokuwa, Y.; Imai, Y.; Muto, S. J. Non-Cryst. Solids. 1988, 105, 198. (34) Iwamura, R.; Higashiyama, N.; Takemura, K.; Tsutsumi, S.; Kimura, K.; Adachi, G. Chem. Lett. 1994, 1131. (35) Chen, H.; Archer, R. D. Macromolecules 1996, 29, 1957. (36) Gao, B. J.; Yang, Y. F.; Cheng, Y.; Shi, D. J. Spectro. Spectra. Anal. 2002, 22, 371. (37) Wang, L. H.; Wang, W.; Zhang, W. G.; Kang, E. T.; Huang, W. Chem. Mater. 2000, 12, 2212. (38) Ding, J. J.; Jiu, H. F.; Bao, J.; Lu, J. C.; Gui, W. R.; Zhang, Q. J.; Gao, C. J. Comb. Chem. 2005, 7, 69. (39) Qiao, X. F.; Yan, B. J. Phys. Chem. B 2008, 112, 14742. (40) Kim, K. M.; Adachi, K.; Chujo, Y. Polymer 2002, 43, 1171. (41) Han, X. X.; Zhou, R. X.; Zheng, X. M.; Jiang, H. J. Mol. Catal. A: Chem. 2003, 193, 103. (42) Chuai, X. H.; Zhang, H. J.; Li, F. S.; Wang, S. B.; Zhou, G. Z. Mater. Lett. 2000, 46, 244. (43) Coltrain, B. K.; Ferrar, W. T.; Landry, C. J. T.; Molaire, T. R.; Zumbulydis, N. Chem. Mater. 1992, 4, 358. (44) Mark, J. E.; Sur, G. S. Polymer Bull. 1985, 14, 325.

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